In eukaryotes, sister chromatid cohesion is established during S phase of cell cycle and maintained throughout G2 until M phase. During mitosis, this cohesion is destroyed along the entire length of chromosome, allowing sister chromatid to segregate to the opposite sides of cell (equational division) and ensuring that each daughter cell receives one copy of each chromosome. In contrast, meiosis consists of two rounds of chromosome segregation following a single round of DNA replication, leading to the formation of four haploid gametes from one diploid germ cell. During meiosis I, homologous chromosomes (homologues) pair to recombine, forming chiasmata in which one sister chromatid from one homologue is covalently attached to a sister chromatid from the other homologue. Hence, in order for homologues to segregate at meiosis I, cohesion of sister chromatid is necessary to be dissociated along the chromosome arms to resolve chiasmata. However, sister chromatid cohesion is retained at centromere until meiosis II, and utilizes the residual centromeric cohesion when sister chromatid segregates, in the same manner as it does in mitosis. Thus, meiotic division requires sister chromatid cohesion to be dissociated in two steps. However, the molecular mechanism for protection of centromeric cohesion only during meiosis I and only at the centromere has remained to be elucidated (e.g., see Annu Rev Genet 35, 673-745 (2001)).
There are important clues as to the molecular nature of sister chromatid cohesion, and the mechanism dissociating sister chromatid cohesion at the onset of anaphase (e.g., see Annu Rev Genet 35, 673-745 (2001); Curr Opin Cell Biol 12, 297-301 (2000); Curr Biol 13, R104-14 (2003); Annu Rev Cell Dev Biol 17, 753-77 (2001); Genes Dev 16, 399-414 (2002>>. In various eukaryotes, sister chromatid cohesion depends on a multisubunit cohesin complex including Scc1 (Rad21 in fission yeast Schizosaccharomyces pombe). Anaphase promoting complex (APC)-dependent degradation of the securin, Cut2/Pds1, allows to dissociate the Cut1/Esp1 endopeptidase (separase), which in turn cleaves Rad21/Scc1, dissociating sister chromatid cohesion. During meiosis, the cohesion subunit Rad21/Scc1 is replaced with a meiotic counterpart, Rec8 (e.g., see Cell 98, 91-103 (1999); Mol. Cell. Biol. 19, 3515-3528 (1999); Nature 400, 461-4 (1999); Genes Dev 15, 1349-60 (2001); J Cell Biol 160, 657-70 (2003)). As Rec8 complex resides only at centromere after meiosis I and the depletion of Rec8 destroys centromeric cohesion, the presence of Rec8 at centromere has been thought to confer the persistence of cohesion throughout meiosis I (e.g., see Nat Cell Biol 1, E125-7 (1999)). Several lines of evidence suggest that Rec8 along chromosome arms is cleaved by separase at anaphase I while centromeric Rec8 is specifically protected until metaphase II (e.g., see Cell 103, 387-98 (2000); Embo J 22, 5643-53 (2003)). Budding yeast SP013 has been implicated in the protection of centromeric Rec8 (e.g., see Genes Dev 16, 1659-71 (2002); Genes Dev 16, 1672-81 (2002)), but SP013 is not centromeric and may function indirectly. Drosophila MEI-S332 is a protein residing at centromere, is required for the persistence of centromeric cohesion during meiosis I, and has features of a candidate protector of meiotic centromeric cohesion, although the details of such protection have so far not been elucidated (e.g., see Annu Rev Cell Dev Biol 17, 753-77 (2001); Cell 83, 247-256 (1995)). Despite the completion of genome sequencing projects on several organisms, no homologue of these proteins has emerged, preventing the formulation of a generalized view of the protection. Concurrently, studies in fission yeast have illuminated the importance of pericentromeric heterochromatin for recruiting centromeric Rec8 complexes and ensuring centromeric cohesion during meiosis I (e.g., see Science 300, 1152-5 (2003)). However, pericentromeric heterochromatin cannot alone confer the specific protection of Rec8 at meiosis I toward meiosis II.